The purpose of the study is to determine the effects of a walking exercise program on management of fatigue during breast cancer treatment. The study is a randomized, controlled clinical trial design, stratified by type of medical treatment: radiotherapy, adjuvant chemotherapy, or combined modalities. Three university teaching hospitals with National Cancer Institute-designated Cancer Centers comprise the clinical research settings. The sample will be newly diagnosed Stage I-III breast cancer patients who have completed surgical treatment and are beginning primary radiotherapy or adjuvant cytotoxic chemotherapy or combinations of both. Following random assignment, subjects in the exercise group will be prescribed an individualized, self-paced, home-based walking exercise program that they will maintain throughout their cancer treatment. Subjects in the control group will receive the usual care given during breast cancer treatment (no prescribed walking program). dependent variables will be measured at pretest (baseline before treatment) and at post test (end of radiotherapy or chemotherapy). In addition, symptoms will be assessed at the end of the three weeks of radiotherapy and at the mid-point of chemotherapy. Groups will be compared by multivariate analysis of variance or covariance and repeated measures analysis of variance or covariance (for the variables tested at several points during the treatment). The independent variable is participation in a walking exercise program. The primary dependent variable is fatigue level; additional variables are physical functioning, emotional distress, difficulty sleeping, weight gain, and quality of life. The out-of-pocket costs of cancer care and its relationship to unmanaged symptoms also will be investigated. EXERCISE TESTING AND TRAINING FACILITY ONLY; NO INPATIENT DAYS OR OUTPATIENT VISITS.